The invention relates to a high-voltage generator which is provided with a hybrid insulation and is intended notably for rotating X-ray systems involving a high speed of rotation and also for systems in which the high-voltage generator is integrated with an X-ray system so as to form one unit (a so-called single-tank generator).
Generally speaking, X-ray systems include an X-ray source with an X-ray tube for generating X-rays as well as a high-voltage generator which delivers the high voltage necessary for operation of the X-ray tube.
Low weight is particularly important for rotating X-ray systems as used, for example, in computed tomography apparatus (CT apparatus). Such systems notably operate with a speed of rotation amounting to several revolutions per second, for example, subsecond scanners in which centrifugal forces of 30 G or more may occur.
JP-A-06 111 991 discloses an X-ray system provided with a hybrid insulation which is formed by an-insulating oil for the X-ray tube and an insulating gas under a high pressure for the high-voltage generator. The insulating means used for the high-voltage generator consists of a gas in order to reduce the quantity of oil and hence the weight. It is a drawback of this system, however, that it requires an appropriate pressure vessel which may lead to significant additional costs, and that the thermal loadability, and hence also the electrical loadability, is comparatively low because, in comparison with oil, the gas is capable of discharging heat to a very limited extent only.
Therefore, it is an object of the present invention to provide a high-voltage generator which is capable of generating a significantly higher electric output power, its weight being low nevertheless.
This object is achieved by means of a high-voltage generator which is provided with a hybrid insulation which includes at least one insulating member which is shaped and arranged in such a manner that there is formed a plurality of interconnected flow channels for an insulating liquid, the cross-sections of said channels being proportioned such that, in comparison with other areas, a higher flow speed can be realized in areas with a high field strength and/or a strong development of heat in the operating condition.
A special advantage of this solution consists in that the formation of flow channels results in a significantly higher discharge of heat, that is, a significantly higher thermal effectiveness of the flow-type insulating medium (in areas exposed to particularly high thermal loads).
Moreover, the electric strength (in areas with a particularly high field strength) is increased in that on the one hand the higher flow speed and the configuration of the channels significantly reduces the probability of formation of fiber bridges, being a frequent cause of voltage flash-over, and on the other hand the direct path between the components carrying the high voltage and ground potential is interrupted by the insulating member.
As a result of the foregoing, the power density, that is, the ratio of the (maximum) output power to the size or the weight of the high-voltage generator, can be significantly increased.
Granted, U.S. Pat. No. 5,497,409 discloses a xe2x80x9cradiogenic unitxe2x80x9d consisting of a single-pole X-ray tube and other components that are arranged in a given manner and are enclosed by a liquid cooling medium as well as a synthetic material. The aim is to improve the cooling in conjunction with other steps. However, therein the weight of the unit in relation to the output power and the voltage strength is of no importance; moreover, no use is made of different flow speeds, notably at the area of the high-voltage components of the high-voltage generator.
Advantageous further embodiments of the invention are disclosed in the dependent claims.
The voltage generation in conformity with claim 2 is particularly suitable for an embodiment having a comparatively low output power and offers the advantage that it does not require additional devices such as, for example, pumps.
The embodiment disclosed in claim 3 is particularly advantageous, because on the one hand use can be made of the positive properties of an insulating liquid such as, for example, a high thermal conductivity in the case of flow and a high electric strength as well as self-restoration in the case of a voltage flash-over, while on the other hand the essential drawback of the generally comparatively high weight has to be accepted to a very small extent only, since the quantity of insulating liquid can be reduced significantly further as its effectiveness is enhanced by the flow.
The embodiment disclosed in claim 4 offers the advantages that on the one hand the surface of such a high resistance foam is very smooth, so that the flow of the insulating liquid is hardly impeded whereas on the other hand a very high electric strength can be achieved because of the microporosity.
The embodiment disclosed in claim 5 also enhances the electric strength of the overall system further, because the electric field lines at the interface between the insulating member and the insulating liquid are not interrupted to any significant extent, thus avoiding the inherent negative surface effects such as the formation of space charges as well as tangential components of the field strength along the interface.
The embodiment of claim 6 is particularly advantageous when the main object is to minimize the weight of the high-voltage generator.
The discharging of heat is particularly high in the embodiments in conformity with the claims 7 to 10, so that comparatively high output powers can be realized.
Finally, it is to be noted that the high-voltage generator in accordance with the invention can be combined with practically any type of X-ray source so as to form an X-ray system, because its continuous power can be scaled within broad limits by appropriate configuration in conformity with the above description. This also holds for the use in a computed tomography apparatus for which the high-voltage generator can be proportioned so as to have a particularly low weight.